The present invention relates to a pulley pressure control system for a transmission which controls a pressing force and an elastic force applied to a pulley to stabilize torque and achieve high efficient transmission, which is applicable to general industrial machines, vehicles, electric motors and the like.
A constant horse power transmission is known from U.S. Pat. Nos. 4,973,288 and 5,269,726; the former discloses a hydraulic type and the latter a screw pressurizing type. The inventive concepts of both the patents have a principal defect. FIGS. 1(A) to 1(F) are views of assistance in explaining the principle of transmission in a belt type transmission. The figures show in the order of (A) to (E) change-over-time of a belt 3 on a secondary pulley 2 when a primary pulley 1 supplies a speed reducing instruction to the secondary pulley 2. When a constant speed ratio shifts from ε0 to ε1, the contact radius of the belt 3 keeps a concentric circle; however, the belt 3 generates a skip motion at a final stage as shown the two figures (D) and (E). More specifically, at this time, a gap 3′ is created between the belt and the pulley, with the result of which the application of pressure is instantaneously stopped as shown in FIG. 1(F). The quality of stable transmission in the belt type transmission depends on whether or not appropriate frictional force can be automatically recovered momentarily after the skip motion. In addition, also the quality of quick responsibility depends on the same. The belt type transmissions proposed by the above patents disclose cam compensation; however, the cam is recovered due to the release of pressure upon the skip motion, resulting in inappropriate operation. On the other hand, even if the transmission is artificially controlled only by a pressing force with the use of an automatic control system that involves a time delay inherently, it is realistically impossible to recover the appropriate frictional force momentarily, so that the quick responsibility will not be attained.
The present applicant has proposed in U.S. Pat. No. 6,120,400 frictional force control with the use of an elastic body and the division of roles of pulley function. The pulley function includes a reference pulley function and a follower pulley function by discriminating a pressing force and an elastic force respectively. To be more specific, the reference pulley function implements a rotation speed control and positioning a belt with the pressing force. The follower pulley function implements a torque control by application of frictional force of the elastic force. However, there remain some outstanding problems. First, although the follower function side is able to control the elastic force, the reference pulley function side has only the pressure as a control element, so that the frictional force supplied to the reference pulley function side cannot be positively controlled. Consequently, a shaft torque control cannot be performed sufficiently. On the other hand, the follower pulley side does not include the rotation speed control element, which leads to the same problem. Second, transmission efficiency deteriorates at both end ranges of a speed change range. That is, the transmission efficiency can not be averaged over the entire speed change range, which leads to narrowing an actual speed change range. Since transmission capacity of the belt type transmission will be determined by the product (N×T) of a rotation speed N and torque T for each pulley, the first problem is essentially the same as second problem. Accordingly, it is possible to solve the second problem if the first problem can be solved. More specifically, the control elements of the rotation speed and the shaft torque are configured to be independently adjustable for each pulley. In other words, a regulating function is divided into a function for regulating the rotation speed and a function for regulating the torque. This achieves high accuracy and high efficiency in the transmission.